1. Field of the Invention
This invention relates to an improved braking system for use in fluid braking systems used on the semi-trailers of highway tractor-trailer vehicles.
2. Discussion of the Prior Art
To meet all of the emergency and service requirements for highway tractor-trailer combinations, while meeting all of the pertinent governmental regulations, including Federal Regulation FMVSS 121 (Docket 90-3, Notice 2), the trucking industry has resorted to a number of trailer brake valve arrangements which have led to complexity, high costs, installation and maintenance difficulties and related problems.
FMVSS 121 is an important safety standard that applies to highway tractor-trailer vehicles; it requires that the spring brakes, which are spring loaded to engage the brake drums when the braking system air pressure drops below the operating pressure necessary to operate the service brakes, become engaged within certain specified time periods. This provides emergency braking when the service brakes become inoperative.
In a typical semi-trailer braking system, a spring brake valve system will be employed to supply pressurized air to the spring brake chambers to release the spring brakes, and also to exhaust air from the spring brake chambers when the spring brakes are to be again applied. Two prior art braking systems are shown in FIGS. 1 and 2. These braking systems are shown generally at 10 in FIG. 1 and generally at 110 in FIG. 2.
The braking system 10 in FIG. 1 comprises a control air line 12, a supply air line 14, a pressure protection valve 16, and a spring brake valve 18. Supply air line 14 is connected to pressure protection valve 16 and thereafter to spring brake valve 18. A line runs from pressure protection valve 16 to a check valve 20. Check valve 20 connects to a service brake reservoir 22. A relay valve 24 is connected to control air line 12 and is operable to supply air to the service brakes 26 from service brake reservoir 22 via line 28. A line also runs from pressure protection valve 16 to a second check valve 30 that is parallel to check valve 20 and its associated line. Second check valve 30 connects to a spring brake reservoir 32. Supply air is provided to spring brakes 34 from both reservoirs 22, 32. Supply air is provided from reservoir 22 via check valve 38 through line 36 leading to the spring brake valve 18. Supply air is provided from reservoir 32 via check valve 40 which connects to line 36 and spring brake valve 18. These valve components may be housed within a single housing as schematically illustrated by the dashed line surrounding the components. Supply air is available to the spring brakes 34 when there is sufficient air pressure in supply air line 36 to open spring brake valve 18.
The braking system 110 in FIG. 2 comprises a control air line 112, a supply air line 114, a pressure protection valve 116, and a spring brake valve 118. Supply air line 114 is connected to both pressure protection valve 116 and to spring brake valve 118. A line runs from pressure protection valve 116 to a check valve 120. Check valve 120 connects to a service brake reservoir 122. A relay valve 124 is connected to control air line 112 and is operable to supply air to the service brakes 126 from service brake reservoir 122 via line 128. Check valve 120 also connects in series to a second check valve 130. Second check valve 130 connects to a spring brake reservoir 132. Supply air is provided to spring brakes 134 via line 136 and the spring brake valve 118 when there is sufficient air pressure in supply air line 136 to open spring brake valve 118.
It has been found that the brake systems shown in FIGS. 1 and 2 have a serious operating problem that arises as a result of the inclusion of the pressure protection valve 16, 116 with the check valves 20/30, 120, a valve combination which is essentially mandated by FMVSS 121. This is the problem of partial release of the spring brakes, sufficient to permit the tractor-trailer combination to be moved, but insufficient to fully release the spring brakes. This condition occurs when the initial start up and charging of the tractor brake system provides sufficient pressure to supply the tractor reservoirs to a minimum operating pressure (typically about 105 psig), and the operator then releases the tractor and trailer spring brakes and drives off. Often the tractor's compressor does not thereafter provide sufficient air pressure to open trailer pressure protection valve 16 and permit additional charging of the trailer reservoirs. In other circumstances, the pressure on the reservoir side of the check valves 20/30, or 120, may prevent additional charging of the trailer reservoirs by raising the effective pressure of pressure protection valve 16, 116. The insufficient trailer reservoir air pressure causes the spring brakes to only partially release, and drag, with potential for wheel fires, excessive brake wear, drive line wear, tire wear, and poor fuel economy.
It would be desirable if a brake system were configured to prevent release of the spring brakes until there is sufficient air pressure in the reservoirs to insure complete release of the spring brakes.